Light guide device enhancing a polarized component and liquid crystal display device

ABSTRACT

A polarized component is obtained with a high conversion efficiency in a light guide which produces one of the polarized components by having it transmitted. The light from a light source is incident to a light guide which comprises a plurality of light guide layers and reflected by the end surface to an interface between the light guide layers. The polarized component transmitting through the end surface is rotated in its polarization plane by a wave length plate and reflected by a reflecting plate for reentrance to the light guide at the end surface of the light guide toward the interface. The reentering light mostly transmits through the interface because the polarization plane is rotated. A reflected light polarized component is returned to the wave length plate and the reflecting plate, and directed back to the interface again. The polarized component transmitting through the interface is similarly transmitted and reflected in the next interface. The number of interfaces can be reduced by increasing the reflection of the polarized component reflected in the interface. For this purpose, the index of refraction in the direction along the axis of the reflected polarized component is increased by making the index of refraction of the light guide layer anisotropic.

This is a division of application Ser. No. 09/399,124, filed on Sep. 20,1999 U.S. Pat. No. 6,118,503.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light guide unit for use in a liquidcrystal display device in which a polarized component of light isenhanced and a liquid crystal display device which is provided with suchlight guide unit. Particularly, this invention relates to a light guideunit for efficiently converting the light from a light source to apolarized light and a liquid crystal display device having means forefficiently directing the polarized light emitted from such light guideunit to a liquid crystal cell.

2. Description of the Related Art

A liquid crystal display device is conventionally observed by directingpolarized light to a liquid crystal cell to cause the polarization planeto be rotated depending on the condition of the cell for passage througha polarizer plate. A light source of the polarized light is placed inthe back of the liquid crystal plate and thus is called a “back light”.For obtaining such polarized light wave, a non-polarized light wasconventionally incident to a polarizer plate and either one of thepolarized components; i.e., S component and P component, was absorbed.

Assuming that a plane defined by a light incident to a point ofincidence on a surface is an incident plane, a polarized componentparallel to the incident plane is called a P component while a componentperpendicular to the incident plane is called an S component. Therefore,more than 50-percent of an incident light was not effectively utilizedin principle and an actual measurement shows that about 58-percent ofthe incident light is absorbed.

Further, a light dispersing sheet having printed dots was typically usedin addition to a polarization device for obtaining polarized light byabsorbing a polarized component in a conventional Liquid Crystal Display(LCD) device, and this makes an additional 20-percent of the lightunavailable.

In FIG. 1, a LCD module 100 of a conventional LCD device is shown. Thelight emanating from a light source 101 transmits through a light guideplate 102 having 96% transmittance, a dispersion sheet 103 having 80%transmittance, a lower polarizer plate 104 having 42% transmittance, aglass substrate 105 having a numerical aperture of 40%, a color filter106 having 30% transmittance, and an upper polarizer plate 107 having90% transmittance, resulting in an actually available light intensitywhich is 3.5% of the light generated in the light source 101. Thisgreatly prevents the energy from being utilized efficiently.

A back light system of a high intensity for use in a low powerconsumption LCD device is especially desired because it is an importantobjective in a portable personal computer to assure a longer usable timewith a given capacity of a battery and the power consumption of a backlight 108 is a major percentage of total power consumption.

Also, the light energy absorbed in the lower polarizer plate 104, etc.,is converted to heat energy which contributes to degradation of parts ofthe LCD device. Particularly for a liquid crystal material of STN (SuperTwisted Nematic) type in which the display quality is degraded by heat,it is an important objective to reduce such heat generation. As seenfrom FIG. 1, 66.4% of the light energy is converted to heat energy bythe light absorption in the lower polarizer plate 104 and the dispersionsheet 103 (this is 69% of heat generation by the light energy).

In order to solve such problems, the applicant of this application filedJapanese patent application no. 9-249139 relating to a method ofimproving the efficiency of light utilization in obtaining a polarizedlight by making available for use at least a part of a polarizedcomponent which had not been utilized. The principle of this method isshown in FIG. 3.

Light from a fluorescent lump CFL which is a light source is incident tothe end surface of a laminated light guide plate unit via a reflectingmirror and a collimator. It propagates through the layers of the lightguide plates, and arrives at the other end surface which is cut in anangle. The incident light is partly reflected at the other end surfacewith the rest being transmitted therethrough. The polarization plane ofthe light transmitting through the end surface is rotated by a quarterwave length plate placed thereunder and reflected by a reflecting plateplaced under the quarter wave length plate for reentrance to layers ofthe light guide plate again through the quarter wave length plate as a Pcomponent.

The P component reentering the light guide plates is incident to theinterface with an adjacent light guide plate layer. The angle ofincidence of the light on the interface is the Brewster angle (to bedescribed later in detail). Therefore, all the P component and a part ofthe S component of the light incident to the interface transmit throughthe interface with the rest of the S component reflected back to thequarter wave length plate and the reflecting plate. The light reflectedagain by the reflecting plate is again directed to the interface afterbeing converted to a P component by the quarter wave length plate whereall the P component and a part of the S component, if any, transmit withthe rest being reflected.

The light reflected here is reflected repeatedly in a similar manner anda light converted to a P component for each reflection transmits throughthe interface. As such, the light guide unit ultimately emits a largeportion of the light from the light source as a P component. Thepolarized light is emitted in the direction largely deviated from thenormal to the front. A prism sheet for redirecting the light to thefront toward the liquid crystal cell is used. The polarization can befurther improved by placing a further polarization plate on the prismsheet.

Because the reflectance and transmission characteristics are differentbetween the S component and the P component, the light transmittingthrough the interface and the light reflected by the interface havedifferent polarization components. To explain the principle of operationof this invention, a change of polarization components of the light intransmitting through or reflecting from the interface between materialsof different indices of refraction is described with reference to FIGS.4, 5 and 6.

In FIG. 4, when light 204 reaches an interface 203 between two materials201 and 202 having different indices of refraction n₁ and n₂,respectively, a part of the light 205 is reflected when the angle ofincidence φ₁ is less than a critical angle while a part of the light 206transmits through the interface. Assuming that a plane defined by alight incident to a point of incidence on a surface is an incidentplane, the incident light 204 is divided into a P component parallel tothe incident plane and an S component perpendicular to the incidentplane.

Modifying Maxwell equation for a dielectric material, the transmittanceof the polarized components P and S are given by;

Tp=sin (2φ₁)×sin (2φ₂)/(sin² (φ₁+φ₂)×cos² (φ₁−φ₂))

Ts=sin (2φ₁)×sin (2φ₂)/sin² (φ₁+φ₂)

dn ₁×in (φ₁)=n ₂×sin (φ₂)

where Tp: transmittance of P component (1−reflectance Rp)

Ts: transmittance of S component (1−reflectance Rs)

φ₁: incident angle of light

φ₂: exit angle of light

n₁: index of refraction of material 201

n₂: index of refraction of material 202

or it is known that;

Rp=((n ₁/cos φ₁ −n ₂/cos φ₂)/(n ₁/cos ∠₁ +n _(2/cos φ) ₂))²

Rs=((n ₁×cos φ₁ −n ₂×cos φ₂)/(n ₁×cos φ₁ +n ₂×cos φ₂))²

where

Rp: reflectance of P component (1−transmittance Tp)

Ts: reflectance of S component (1−transmittance Ts)

The reflectance of the P polarized component and S polarized componentvary depending on the incident angle φ₁ and the exit angle φ₂ as shownin FIG. 5 and FIG. 6, and differ from each other even in a same incidentangle φ₁ (reflectance/transmittance characteristics are differentbetween S and P polarized components).

For example, when the light proceeds from an acrylic material having anindex of refraction of 1.49 to air which has an index of refraction of1.00 (FIG. 6), the critical angle in which a total reflection takesplace is 42.1-degrees. If the light is incident at 40-degrees which isless than the critical angle, the exit angle φ₂ will be 77.8-degreesaccording to Snell's law. Substituting the above equation of Rs and Rpwith this, the reflectance for the S component is 35.69% while thereflectance for the P component is 7.98%.

It should be clearly understood from the above description referring toFIGS. 4 to 6 how the polarized components of the light are transmittedand reflected in the interface in this invention.

It is understood from the above-described principle that it is importantfor the layers of the light guide to be laminated in multiple layers tocause the unnecessary S component to be reflected back each time thelight reaches the interface between the layers and to be returned as a Pcomponent for transmitting through the interface thereby improving theefficiency of converting the light emitting from the unit eventually toa P component.

However, it is disadvantageous to laminate too many layers from the viewpoint of the efficiency of utilizing the energy of the light sourcebecause each layer invites some loss of light. In addition, theincreased number of laminated layers would result in the increase of thethickness of the entire unit even if a thin layer is used. The increaseof the thickness would also invite an increase of the weight. It is themost important objective for a portable information processing device,such as a notebook computer, to decrease the power consumption of itsbattery as well as the thickness and the weight of the entire unit asmuch as possible.

SUMMARY OF THE INVENTION

This invention relates to an improvement of a light guide unit of theabove-described type, and it is an object of this invention to provide alight guide unit having an unchanged performance with a decreasedthickness of the entire unit.

It is another object of this invention to improve the brightness of aliquid crystal display device without resulting in an increase of powerconsumption by efficiently combining the polarized light from such lightguide unit of a high efficiency to a liquid crystal cell.

The basic configuration of this invention lies in a structure in whichthe light from a light source incident to an end surface of a unit oflaminated light guide plates propagates through each layer of the lightguide plates and is partly reflected by the other end surface which isobliquely cut. The rest of the light transmitting therethrough causesthe polarization plane of the transmitting light to be rotated by a wavelength plate lying thereunder and reflected by a reflecting plate lyingunder the wave length plate for reentrance to the light guide plateagain through the wave length plate as a P component.

The P component reentering the light guide plate is incident to aninterface between neighboring light guide plates. The incident angle ofthe light incident to the interface is adapted to be the Brewster angle.Therefore, all P component light incident to the interface and a part ofthe S component light transmit the interface while the rest of the Scomponent light is reflected back to the wave length plate and thereflecting plate. The light reflected again by the reflecting plate isdirected back to the interface after being converted to a P component bythe wave length plate, and all P components and a part of S component,if any, transmit through the interface while the rest is reflected.

The light reflected here is subject to the same process repeatedly, anda light converted to a P component in every repetition transmits throughthe interface. As such, the light guide unit eventually emits a largeportion of the light from the light source as a P component. Because thepolarized light is emitted in the direction largely deviated from thenormal to the front, a prism sheet for redirecting the light to thefront toward the liquid crystal cell is used.

In this invention, it is important in the principle of this inventionthat the S component is reflected in the interface of the light guidelayers. The number of the interfaces; i.e., the number of the lightguide layers can be reduced by causing as much S component as possibleto be reflected to reduce the S component transmitting through theinterface.

This invention provides a conversion efficiency comparable to lightguide layers using an isotropic material with a lesser number of lightguide layers by using a material of an anisotropic index of refractionas the light guide layers to improve the reflectance of the S componentin the interface. The axes of two indices of refraction of theanisotropic material coincide with the planes of P and S components,respectively. While the index of refraction in the direction of the axislying in the plane of the P component may be the same as a conventionalone, the index of refraction in the plane of the S component is higherthan the conventional one. The higher, the better. It is seen from theexpression of the reflectance Rs described above that the reflectance ofthe S component becomes larger when the index of refraction in the axisof the plane of the S component in the interface is larger.

The light guide unit comprising laminated light guide layers of suchanisotropic index of refraction receives an incident light from a lightsource at the end surface thereof which is a cross-section of thelaminated layers to cause a part of the incident light to be reflectedat the opposite end surface which is obliquely cut and the rest of thelight to be transmitted therethrough. A quarter wave length plate isattached to the obliquely cut end surface and a reflecting plate isprovided under the wave length plate.

The light transmitting through the end surface is reflected by thereflecting plate after being rotated by the wave length plate and isincident to the end surface after being rotated by the wave length plateagain. The light incident to the end surface is incident to theinterface where it is transmitted and reflected as described herein.However, the majority of the S component is reflected in the interfacewith the rest transmitting through the interface in this invention.Therefore, the light from the light source can be converted to the Pcomponent with a lesser number of layers.

In this invention, it is preferred that the light incident to the firstinterface of the light guide is incident in the Brewster angle. It isreadily seen by drawing a geometrical drawing that the angle ofincidence of the light to the obliquely cut end surface of the lightguide unit decides the angle of incidence at the interface. In thisinvention, the angle of incidence of the light to the obliquely cut endsurface of the light guide unit is so adjusted that the light incidentto the first interface of the light guide is incident in the Brewsterangle.

The light guide unit is so inclined with respect to the wave lengthplate and the reflecting plate as to provide an incident angle decidedin this manner. In order to reduce the inclination, a plurality ofslopes making such incident angle may be formed in the obliquely cut endsurface. This allows a necessary incident angle to be provided withoutinclining the entire light guide unit in this angle. This allows thethickness of the entire unit to be further reduced.

In this invention, the light guide unit may be formed into a shape of atriangular wedge consisting of the top layer of the laminated layers,the obliquely cut end surface and the surface to which the light fromthe light source is incident. This allows a wedge-shaped space to beprovided under the unit for receiving various components. This isadvantageous for a portable data processing device in which a thin andlight weight type is especially desired.

In another aspect of this invention, the prism means for directing thepolarized light to the front has a plurality of prisms disposed in asame pitch as columns of the liquid crystal cells. Each prism has anincident surface and a reflecting surface. Because the light is emittedfrom the reflecting surface, a portion corresponding to the incidentsurface is dark. In this invention, the dark incident surface portion isso disposed as not to contribute illuminating the liquid crystal cell byhaving the portion corresponding to the reflecting surface align thecolumns of the liquid crystal cell. All the polarized light emitted fromthe light guide is thus directed to the liquid crystal cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional LCD device.

FIG. 2 is a diagram showing the structure of a conventional LCDpolarizer plate unit.

FIG. 3A is a diagram showing the structure of a conventional LCDpolarizer plate unit.

FIG. 3B is an expanded view of a segment of FIG. 3A.

FIG. 4 is a diagram showing refraction of light between differentmaterials.

FIG. 5 shows a characteristic plot of reflectance when the light isincident from a material having an index of refraction of 1.0 to amaterial having an index of refraction of 1.49.

FIG 6 shows a characteristic plot of reflectance when the light isincident from a material having an index of refraction of 1.49 to amaterial having an index of refraction of 1.0.

FIG. 7 is a diagram showing deflection of the light by a prism sheet.

FIG. 8 is a diagram showing deflection of the light by a prism sheet.

FIG. 9 is a schematic diagram showing a concept of another embodiment ofthis invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic structure of this invention is shown in FIG. 2. The laminatedlight guide unit is made of thin light guide layers laminated as shownand a light source is attached to an end surface thereof. The lightsource comprises a fluorescent lamp and a reflecting sheet. Thelamination is cut so as to assume an oblique end surface to which acombination of a quarter wave length plate and a reflecting plate isaffixed.

The end surface makes an angle φ with respect to the quarter wave lengthplate and the reflecting plate. Therefore, the light is incident to theend surface at an angle of π/2−2φ. It is readily found from ageometrical drawing that an incident angle to the interface betweenlayers is π/2−2φ.

It is preferable that this incident angle is the Brewster angle. Whenthe light is incident to the interface at the Brewster angle, allpolarization component lying in the incident plane (P) transmits theinterface while all polarization component lying in a plane orthogonalto the incident plane (S) is reflected. Any S component which maytransmit through the interface will be reflected by the next interface.The S component reflected from the interface is returned back to thelight guide layers by the wave length plate and the reflecting plate asa P component and incident again to the interface. The S component isreflected each time the above process is repeated at a plurality ofinterfaces transmits through the interface as a P component so that alarge portion of the light from the light source is emitted from thelight guide unit as a P component.

A plurality of thin light guide films may be further laminated on thetop light guide layer of the light guide unit as shown in FIG. 3. Thisfurther adds interfaces of transmission and reflection.

The light guide member and the plurality of light guide layers arepreferably of a material which assumes a low internal absorption of thelight, such as an acrylic sheet and preferably transparent materialsincluding acrylic resin, PMMA (polymethylmethacrylate), polycarbonate,polyethylene, Se, and AgCl. The shape of the light guide member may bein a shape suitable for use such as a bar and a curved surface withoutbeing limited to a plate and a sheet.

The light guide member may be of a single piece or a lamination of aplurality of sheets. These light guides are not limited to a same sizeor a same material and a member requiring stiffness may be designedthick while a member which does not require stiffness may be designedthin. Also, materials of differing indices of refraction may bedeposited in multiple layers on a stiff light guide to increase thenumber of laminated layers while maintaining a stiffness.

In using an acrylic sheet in the light guide member, the thickness ofthe sheet is preferably 0.1 to 4.0 mm from the consideration of thestiffness and the efficiency of light utilization. The lamination asused in this invention is not limited to insertion of air between thelight guides and water vapor may be introduced between the guides forpreventing degradation of the light guide unit, water or an adhesive maybe inserted between the guides for preventing the guides from beingpeeled off, or a material having an index of refraction differing fromthe light guide may be inserted. Higher reflectance of the reflectingplate is preferable in this invention and the reflecting plate may bemade of an aluminum deposited sheet, a silver deposited sheet and ametal foil, etc.

In this invention, the light guide layer is made of a material having ahigh index of refraction in the axis lying in the plane of the Scomponent. For example, while the isotropic index of refraction of anacrylic material is normally 1.49, the index can be increased up toabout 1.69 in the direction of the axis lying in the plane of the Scomponent. By doing so, an increased portion of the S component isreflected in the interface (lesser amount of the S component transmitthe interface) so that an unchanged effect can be resulted with a lessernumber of layers than those required for an isotropic material.

For example, when an acrylic material having an index of refraction 1.49is used as an isotropic material, the reflectance of the S component is28% while the transmittance is 72% per layer. With ten layers laminated,the overall transmittance will be 0.72¹⁰=0.04.

On the other hand, if the index of refraction in the direction of the Scomponent is 1.66, the reflectance is 40% while the transmittance is 60%and the same effect is obtained with 6 layers (0.6⁶=0.04). A sheethaving such anisotropic index of refraction is easily available in themarket.

While the thickness of the light guide film is not important and it ispreferable that the number of the interfaces is as large as possible,the light guiding layer is preferably as thin as possible from the viewpoint of reducing the weight of the light guide unit. An extra space iscreated by making the thickness of the light guiding layer in thisportion extremely thin and the layers of substantially same size may beused in lamination without requiring the layers to be progressively indifferent sizes resulting in a stepped structure as shown in FIG. 2.

As such, the light is not lost by re-entering from the edge of thelayers and dark and bright stripes are eliminated. Even if the stepsremain in the layers as shown in FIG. 2, there is little possibility ofthe light re-entering and recognizable stripes are not generated becausethe layer is thin and the size of the edge is very small.

By employing the above structure, this invention allows thecross-sectional shape of the light guide unit to be of a triangularshape as shown in FIG. 3, in contrast to the conventional unit which hada rectangular cross-section as shown in FIG. 2. By this structure, theweight and the volume of the unit can be about half the conventionalunit. Also, this invention can implement a mode which is similar to themode in which a conventional back light (not generating a polarizedlight) uses a light guide of a wedge shaped cross section to provide aneffective use of a space and allows a conventional back light to bereplaced with the present polarized back light in a form compatible withthe conventional type.

While the light guide layer is acrylic material and the surroundingmaterial is air in the example so far described, any material of thelayer and any surrounding material may be used so long as the indices ofrefraction of the materials allow the incident light to satisfy theBrewster angle or an angle which is near the Brewster angle.

The following condition is required for the incident angle π/2−2φ to bethe Brewster angle θ_(B). In the expression, n₁ is an index ofrefraction of the light guide, n₂ is an index of refraction of amaterial other than the light guide (air in FIG. 5), and φ is the angleof the groove (the slope of the larger angle of inclination). Therelationship between Brewster angle θ_(B) and n₁, n₂ is given by;

θ_(B)=sin⁻¹ [n ₁ ²/(n ₁ ² +n ₂ ²)]^(½)(rad)

The angle of incidence to the upper surface of the light guide is givenby a geometric analysis using φ;

π/2−2φ (rad)

Snell's law is expressed on the upper surface of the light guide as;

sin θ_(B)/sin (π/2−2φ)=n ₁ /n ₂

solving this expression for φ gives the following general solution;

φ=cos⁻¹ {(n ₂ /n ₁)[n ₁ ²/(n ₁ ² +n ₂ ²)]^(½)}/2(rad)

Any medium satisfies the condition of this invention so long as itsatisfies the above general expression.

While the entire light guide unit is inclined with respect to the wavelength plate and the reflecting plate so as to provide an incident anglewhich is equal to the Brewster angle, many sloped surfaces which providesuch incident angle can be formed in the obliquely cut end surface. Asshown in the enlarged view in FIG. 3, many sloped surfaces runningperpendicularly to the face of the drawing are formed in the obliquelycut end surface and are so disposed as to provide a desired angle to theincident light in the light guide. An incident angle satisfying theBrewster angle is thus provided though the entire light guide is notinclined in this angle. A necessary incident angle can be thus providedwhile the light guide unit is not entirely inclined in this anglethereby reducing the thickness of the entire unit.

This invention is contemplated for use as a back light of a liquidcrystal display device. The liquid crystal display device comprises alight source and glass substrates sandwiching a liquid crystal to whicha polarized light emitted from the light guide unit of this invention isincident.

The light emitted from the light guide is largely inclined in 70-degreesfrom the front thereof in this invention. Two methods are available fordeflecting the light to the right angle to the front surface. The firstmethod is to have the light refract twice to deflect it to the front, inwhich a prism sheet is used with the apex thereof oriented upward asshown in FIG. 8. When the index of refraction n of the material of theprism is 1.58, a prism sheet having an angle of apex of 32-degrees isrequired to deflect the light to the front.

A second method is to have the light refract once and totally reflectonce to deflect to the front, in which the prism sheet is used with theapex thereof oriented downward as shown in FIG. 9. In this case, a prismhaving an angle of apex of 65.4-degrees is required. As seen in theabove, a same effect is resulted whether the prism is oriented upward ordownward. From the view point of fabrication, it is more advantageous inthe view point of yield and cost to use the prism with the apex orienteddownward because a smaller apex angle of a prism is more difficult tofabricate (a larger apex angle can be used when the apex is orienteddownward). The prism sheet is made of a glass or plastic material.

In FIGS. 7 and 8, it is seen that the sloped surface of each prism whichis not the light reflecting surface does not emit the light to thefront. In other words, the light emitted from the prism sheet is in astripe pattern. This may possibly induce an interference pattern with agate line or a data line of the liquid crystal cell. In order to preventthe stripe pattern from being generated, the pitch of the prisms of theprism sheet (50 microns, for example) can be made smaller than the pitchof the liquid crystal cell (200 microns, for example) to mismatch thepitches. By doing so, the prism sheet is observed as if it emits thelight uniformly from the front and the interference pattern can beprevented from being generated because the pitch of the prism sheet isvery small.

However, the light incident to a portion of the liquid crystal cellarray which has no opening is absorbed there and wasted in this case.Another aspect of this invention provides a structure in which suchwaste is avoided.

According to this structure of this invention, the pitch of the prismsof the prism sheet is made the same as the pitch of the liquid crystalcell array so that the opening part of the liquid crystal cell coincideswith a portion of the prism corresponding to the reflecting surfacewhich emits the light. The portion corresponding to the slope of theprism which is not the reflecting surface coincides with the part havingno opening.

FIG. 9 is a schematic diagram showing a concept of the inventivestructure. As shown in FIG. 9, the pitch of the prisms of the prismsheet is made the same as the pitch of the liquid crystal cell array.The light reflected by the reflecting surface of the prism is directedto the opening part of the liquid crystal cell. The part having noopening does not receive the light because it faces to the surface whichis not a reflecting surface. All the light emitted from the light guideunit is thus directed to the opening part, and there is no light whichis absorbed without being utilized. It is easy to manufacture the prismbecause the prism has a larger pitch than those shown in FIGS. 7 and 8.The apex angle and the ratio of reflecting/transmitting surfaces may besuitably decided in a specific design work.

As shown in FIG. 9, the liquid crystal cell array may be formed directlyon the prism sheet. In this case, the prism sheet also plays a role of aglass substrate of the liquid crystal cell. The number of interfacesbetween media is decreased by 2 when compared to a case where anindependent prism sheet is disposed between the liquid crystal cell andthe light guide film, resulting in a corresponding improvement ofefficiency.

The thickness and the weight of the light guide are reduced because thelight is converted to the P polarized component with the number oflayers less than those of a conventional light guide according to thisinvention. There is no light which is absorbed without being utilized inanother aspect of this invention because all the light from the lightguide is directed to the opening part of the liquid crystal cell.

The following is a brief description of the reference numbers as used inthe drawings:

100: Conventional LCD device

101: Light source

102: Light guide plate

103: Diffusion sheet

104: Lower polarizer plate

105: Glass substrate

106: Color filter

107: Upper polarizer plate

108: Back light

201: Material 1

202: Material 2

203: Interface between the materials

204: Incident light

205: Reflected light

206: Transmitted light

While the exemplary preferred embodiments of the present invention aredescribed herein with particularity, those having normal skill in theart will recognise various changes, modifications, additions andapplications other than those specifically mentioned herein withoutdeparting from the spirit of this invention.

What is claimed is:
 1. A liquid crystal display device comprising: amatrix array comprising a plurality of liquid crystal cells integrallyformed on a front of a transparent substrate such that said substrateforms a substrate of said plurality of said liquid crystal cells; amatrix array comprising a plurality of prisms integrally formed on aback of said substrate with respect to said matrix array of liquidcrystal cells; each prism within said array matrix of prisms having alight incident surface and a light-reflecting surface; said matrix arrayof prisms having a pitch that is the same as a pitch of said matrixarray of liquid crystal cells; said light-reflecting surface of each ofsaid prisms being aligned with an opening part of each of said liquidcrystal cells, and said light incident surface of each of said prismsbeing disposed to not contribute light to each of said liquid crystalcells; and a source of polarized light illuminating said prisms suchthat said prisms provide all of the polarized light from said source ofpolarized light to illuminate said opening parts of said liquid crystalcells, and such that substantially all of said polarized light isdirected to an opening part of said liquid crystal cells.
 2. A liquidcrystal display device of claim 1 in which said substrate is formed of aglass.
 3. A liquid crystal display device of claim 1 in which saidsubstrate is formed of a plastic material.